Bug 1011000 - Enable audio channel change Gaia UI Tests checking

Acutally I don't really like this wait method being here. I know you adopted it from someone else who put it here but let's take the chance to make the data_layer class have better consistency.
There is no precedent for having a wait in data_layer anywhere else. We only have properties (more or less). 

Can you change this to be just a property of `current_audio_channel` and then move the waits into the actual test? We may not even need to wait for the channel (ie we can just assert it) if we have waited for the UI properly already.